1. Field of the Invention
The present invention relates to the stabilization of metachromatic dyes, and compositions comprising stabilized metachromatic dyes. The present invention is also directed to processes for determining polyionic materials utilizing stabilized metachromatic dye compositions.
2. Discussion of Background Information
Colorimetry is a well-known method of chemical analysis which involves the comparison and matching of a standard color with that of an unknown color to approximate the concentration of a specific component in a sample to be analyzed. When the amount of light absorbed by a given substance in solution is proportional to the concentration of the absorbing species, colorimetry is a simple and accurate method for determining unknown concentrations. For example, if the concentration of a polymer in an aqueous system is to be determined, a sample can be taken, the absorbance of the sample in the presence of a suitable dye can then be measured and compared with a calibration curve to quickly and accurately estimate the concentration of the polymer in the aqueous system. Colorimetry provides advantageous testing since it can easily be performed at the application site. Certain dyes undergo a unique color change upon interaction with polyionic compounds in solution known as metachromasy. Thus, metachromatic dyes are those which undergo a color change upon interaction with polyionic compounds. Any metachromatic dye can be used in a calorimetric test to determine the concentration of a substance, including polycarboxylates, sulfonates, and the like in an aqueous solution. More specifically, when anionic polymers contact a metachromatic dye, the dye molecules align with the anionic charges on the polymers, resulting in a shift in the wavelength of maximum absorbance of the dye molecule. This shift is observable as a color change in the solution containing the dye and the polymer. Thus, since polycarboxylates and sulfonates, which are anionic, induce a metachromatic change in certain dyes, their concentrations in aqueous solutions can be determined calorimetrically by measuring the absorbance, at a specified wavelength, of a solution containing polycarboxylates and/or sulfonates and a metachromatic dye and comparing this absorbance to absorbances of standards having known concentrations of the species being measured.
However, when metachromatic dyes are dissolved in aqueous solutions for use in analytical determinations, fresh samples must be prepared on a daily basis to insure accurate analysis. Aqueous solutions of metachromatic dyes are extremely susceptible to degradation due to a variety of factors, such as light, temperature, dissolved oxygen, pH, etc. For example, when pinacyanol chloride is dissolved in an aqueous solution at a concentration of 9.0.times.10.sup.-5 molar, the pinacyanol chloride degrades at a rate of approximately 10 to 20% per week. Because of this degradation problem, frequent reagent preparations must be made in the field and this is not practical.
Still further, the instability of known dye solutions leads to disadvantageous results associated with the detection process. These disadvantages result in a lack of reproducibility of results, i.e., consistency of results is difficult to attain on separate days even with the same water sample.
Accordingly, there is a need to provide stable metachromatic dyes that enable simple tests for materials, particularly water treatment polymers, and especially enable simple tests that can be utilized over extended periods of time without the need for formulation in the field.